1. Field of the Invention
This invention relates to an adaptable method and apparatus for conditioning the output of a sensor to a desired output transfer function. In one form, the present invention relates to a method for linearizing the output of a sensor which behaves in a nonlinear fashion to changes in a physical quantity being measured.
2. Description of the Relevant Art
The use of sensors in a variety of control applications is increasing at a dramatic rate. For example, in many fields such as robotics, biomedical, and manufacturing, sensors are used to provide feedback to a control system as to position, displacement, temperature, force, or acceleration. The use of noninvasive sensors in which the sensor does not affect the physical quantity being measured is increasing.
A fundamental difficulty in using sensors is that the sensor output must be predictable and desirable for the physical quantity being measured. Typically, sensors are fabricated using costly and tightly controlled manufacturing techniques to produce a sensor with known performance characteristics. The sensor manufacturer analyzes and publishes the operating characteristics and tolerances of the sensor, but cannot usually adequately account for the many variables which affect sensor operation.
It is important that the sensor behave not only in a predictable manner, but also in a desirable fashion. In many applications it is desirable that the sensor output bear a linear relationship to changes in the physical quantity being measured. Indeed, the design of many engineering systems relies upon an assumption that sensor output bears a linear relationship to variations in a physical quantity being measured. Unfortunately, many types of sensors do not perform linearly due to either the nature of the inherent physical principle or mobility to control all manufacturing tolerances to ideal specifications.
Many popular types of sensors such as Hall-Effect position sensors, thermistor, capacitive sensors, and strain gauge - are inherently nonlinear. That is, the sensor output follows a nonlinear transfer function in response to changes in the physical quantity being measured. Even sensors which might be categorized as inherently linear have outputs which are sometimes difficult to predict over certain portions of their dynamic range. Therefore, the outputs of both inherently linear and inherently nonlinear sensors may vary in somewhat unpredictable and undesirable fashion over certain dynamic ranges of the physical quantity being measured.
A variety of techniques have been developed to attempt to overcome the problem of sensors nonlinearity. One such method is to employ an analog compensation circuit which applies an inverse transfer function to the output of the sensor to achieve a linear characteristic. Some software methods have followed a similar approach in applying an inverse transfer function to the sensor output to achieve a system linearization. See, e.g., U.S. Pat. No. 4,581,714. A major drawback of such methods is that such compensation techniques require a prior detailed accounting of the input-output characteristic (an experimentally determined transfer characteristic) of the sensor in question and do not account for nonuniformity under certain conditions. Further, analog compensation circuits are often as problematic as the sensor-requiring high-tolerance components which are unstable for certain input conditions and are difficult to design and implement.
Typically, the sensor manufacturer determines and publishes the operating characteristics of the sensor over the normal operating range. Because such operating characteristics must be uniform for all sensors of a given type, very narrow tolerances (and concomitant high reject rate) and costly manufacturing must be applied to sensor fabrication. Whether or not the sensor performance characteristics are generally linear or nonlinear, many users apply analog correcting circuits to the sensor output. Thus, corrections for frequency response distortion, noise, parameter drift, or cross-sensitivity to temperature, magnetic field, or electric field, might be applied over a given operating range to the sensor.
A useful approach to the problem of obtaining the desired and predictable characteristic is to provide a separate ROM based look-up table which maps nonlinear sensor outputs to corresponding values on a linear scale in a microprocessor-based system. Such a ROM-based system must, however, be tailored to a particular sensor and is therefore most useful when used in one time prototype or research applications. If the same ROM based look-up table is intended to operate with multiple production samples of a given sensor type potentially unachievable rigid manufacturing tolerances must be employed when fabricating the sensor to allow one fixed ROM based look-up table used with all production pieces of such a sensor.
These problems are generally discussed in G. Kondraske and R. Ramaswamy, "A Microprocessor-Based System" IEEE Trans. Instr. Meas., Vol. 3, pp. 338-43, Sept., 1986, and is incorporated herein by reference.